The present invention relates to a self-recovering current-limiting device with liquid metal, including electrodes made of solid metal for the connection to an external electric circuit to be protected and several compression spaces which are partially filled with liquid metal.
Soviet Union Patent Publication SU 922 911 A describes a single-pole, self-recovering current-limiting device containing two electrodes made of solid metal which are separated by first insulating bodies which are designed as a pressure-resistant insulating housing. Inside the insulating housing, compression spaces are formed by insulating intermediate walls and second insulating bodies which are arranged therebetween and designed as ring-shaped sealing disks, the compression spaces being partially filled with liquid metal and arranged one behind the other and interconnected via circular connecting channels of the intermediate walls, the connecting channels being filled with liquid metal and arranged off-center. Thus, in normal operation, a continuous, inner conductive connection exists between the electrodes via the liquid metal. In the current-limiting event, the liquid metal is displaced from the connecting channels as a result of the high current density. In this manner, the electrical connection of the electrodes via the liquid metal is interrupted, resulting in the limiting of the short-circuit current. Subsequent to clearing or eliminating the short circuit, the connecting channels refill with liquid metal whereupon the current-limiting device is operational again. The intermediate walls must resist the pressure rise during the vaporization of liquid metal and are composed of high-quality ceramic material having a high temperature resistance and a high erosion resistance to the action of electric arcs. In German Patent Application DE 40 12 385 A1, a current-limiting device having only one compression space is described, and vacuum, protective gas, or an insulating liquid are mentioned as the medium above the liquid level. According to Soviet Union Patent Publication SU 1 076 981 A, the connecting channels of adjacent intermediate walls are staggered relative to each other for improving the limiting characteristics. It is known from German Patent Application DE 26 52 506 A1 to use gallium alloys, in particular GaInSn alloys in contact devices.
The known current-limiting devices are equipped with current-conducting connecting channels of circular cross-section. The once selected, non-changeable opening cross-section of the connecting channels substantially determines the nominal current carrying capacity of the current-limiting device. Consequently, an ex-works current-limiting device is only suitable for just one nominal current range. On the user side, there is no possibility to adapt the current-limiting device to a higher or lower nominal current range. In the known current-limiting devices, moreover, problems occur at the sealing joints between the joined insulating bodies. The ability to creep of liquid metals places high demands on the tightness of the current-limiting devices. Moreover, the known current-limiting devices have the feature of a component-intensive design and considerable assembly effort.
Therefore, an object of the present invention is to provide a self-recovering current-limiting device having liquid metal, the device being relatively inexpensive to manufacture and relatively simply adjustable to a desired nominal current range.
The present invention provides a self-recovering current-limiting device including a liquid metal. The device includes a first and a second electrode for connection to an electric circuit to be protected, each of the first and second electrodes being made of a respective solid metal. A plurality of pressure-resistant insulating bodies and a plurality of insulating intermediate walls are provided. The plurality of insulating intermediate walls and the plurality of pressure-resistant insulating bodies define a plurality of first compression spaces, the plurality of first compression spaces being disposed one behind the other between the first and second electrodes and being at least partially filled with the liquid metal. The plurality of insulating intermediate walls define a plurality of connecting channels as upwardly open slotted holes, the plurality of insulating intermediate walls being integrally connected to the bottom part. A cover is provided configured for tightly closing the bottom part using at least one of a nonpositive-locking and positive-locking connecting device, the cover and the bottom part forming a housing.
The current-limiting device according to the present invention needs only few components, namely two electrodes, a two-part molded housing serving as an insulating body as well as connecting devices, which are known per se, such as screw, clamping, adhesive, welded or compression connections for connecting the bottom part and the cover. Because of this, the outlay in terms of the provision, assembly and sealing of the remaining parts is equally considerably reduced. The creeping of liquid metal no longer constitutes a problem. The cover and the bottom part, including the intermediate walls, are composed of heat-resistant material, for example, heat-resistant thermoplastic or thermosetting material, mica, or ceramics. The upwardly open connecting channels configured as slotted holes allow the bottom part to be easily removed from the mold. By filling in a certain amount of liquid metal, a certain partial cross-section of the filled connecting channels is determined as the current-conducting cross-section in the horizontal position of use of the current-limiting device, thereby determining the nominal current range. The nominal current carrying capacity increases with the filling height. The filling height can be determined, adapted or changed on the manufacturer or on the user side. This permits optimum adaptation to the conditions of the systems to be protected, involving a minimum number of sizes of a type series of current-limiting devices.
The alternatingly staggered arrangement of the connecting channels which are in each case assigned to one of the intermediate walls results in a current path in meander form and in an elongation of the electric arc in the current-limiting event.
It serves the mechanical stability if both the exterior walls and the intermediate walls are tightly connected to the cover, with the wall heights advantageously being identical. It is recommendable to use a sealing device which may include injection-molded or otherwise molded seal or seals.
To ensure that the bottom part can be easily removed from the mold, it is expedient for the relevant edges and wall surfaces to be slightly slanted.
In the case of connecting channels having the usual slotted hole shape, a linear dependency between the filling height of the liquid metal and the current-conducting cross-section exists. In contrast, this dependency is progressive in the case of a slotted hole shape that markedly widens upward in a conical manner, which can be used for markedly extending the nominal current ranges.
The electrodes can be supported in the bottom part in a simple manner using devices which are known per se and fixed by installing the cover. The assembly effort is reduced if the electrodes are fixed in the bottom part, preferably simultaneously with the molding process of the bottom part. A full-surface coverage of the relevant inner surfaces by the electrodes makes it easier for them to be fixed and serves the stability of the current-limiting device.
A reclosable filling opening at a suitable location of the molded housing allows the current-limiting device in a simple manner to be filled, decanted, or adapted to the required nominal current range with regard to its filling height on the factory or on the customer side. GaInSn alloys as the liquid metal to be used are easy to handle because of their physiological harmlessness. An alloy of 660 parts by weight of gallium, 205 parts by weight of indium, and 135 parts by weight of tin is liquid from 10xc2x0 C. to 2000xc2x0 C. at normal pressure and possesses sufficient electrical conductivity.
The current-limiting device described above as a single-pole device can advantageously be expanded by arranging substantially identical current-limiting devices side by side to form a multipole device. A multipole current-limiting device of that kind expediently possesses a common bottom part having conducting paths which are insulated against one another according to the number of poles and/or a common cover.